1. Field of the Invention
The present invention relates to a knock control device for an internal combustion engine, and more particularly to a knock control device for an internal combustion engine for detecting an occurrence of knocking in the internal combustion engine, based on an ionic current which is generated by combustion in the internal combustion engine, and correcting an internal combustion engine control amount so that the knocking is suppressed.
2. Description of the Related Art
Up to now, in knock control devices for an internal combustion engine, in order to minimize damage caused to the engine by the knock generation, a control amount of the internal engine is controlled so that the knocking is suppressed (e.g., ignition timing is retarded) in accordance with the knock generation.
Further, in an internal combustion engine knock control device for using an ionic current generated immediately after ignition inside a combustion chamber of the internal combustion engine, since there is little difference in knock detection sensitivity of each cylinder, knock control can be performed effectively for each cylinder. A variety of these have been proposed up to now.
Generally, in the internal combustion engine, air and fuel (an air-fuel mixture) introduced into the combustion chamber is compressed by a upward movement of a piston, and a high voltage is applied to a spark plug inside the combustion chamber to burn the air-fuel mixture with electrical sparks generated at the spark plug, whereby a force that pushes down the piston is taken out as an output.
At this time, when the combustion takes place inside the combustion chamber, electrons of molecules inside the combustion chamber dissociate (ionization). Therefore, when the high voltage is applied to the spark plug (an ionic current detection electrode) inside the combustion chamber, movement of ions through the spark plug allows an ionic current to flow.
It is known that the ionic current varies sensitively based on pressure fluctuation inside the combustion chamber, and the ionic current contains vibration components that correspond to the knock generation. Therefore, it is possible to determine the presence or absence of the knock generation based on the ionic current.
In such a type of device, in order to prevent erroneous knock detection due to noise superimposed in the ionic current, a background level is set regarding an ionic current detection signal. For example, in a device described in JP 10-9108 A mentioned above, a signal is generated by performing waveform shaping processing and the like on a knock current detection signal, and for the signal thus generated there is set a background level (a noise level determination reference) which is calculated from a sum produced by adding an average value of detection signal strength to a dead zone (an offset value) corresponding to an operating region.
However, the device described in JP 10-9108 A achieves the knock control based on the ionic current, but it is not provided with correction means for correcting the knock detection and the knock control in a case where additives are mixed into the fuel, and a case where a non-standard spark plug is mounted in the combustion chamber. Therefore, there has been a problem in that fluctuation in the intensity of the ionic current detection signals is likely to cause erroneous knock detection and non-detection.
In order to overcome this problem, JP 2001-82304 A describes a device which is a knock control system similar to the device described in the above-mentioned JP 10-9108 A, wherein fluctuation of the ionic current amounts is detected by performing processing to obtain an average value of integral values of the ionic currents, and then the learned result is used to correct the background level and the like to solve this problem.
Further, as a method of setting the correction amount, the result learned by performing the averaging processing is compared with a comparison level that is set based on R.P.M. and load, and a background level correction amount is set according to a ratio or the deviation between the result learned from the average processing and the comparison level.
As described above, JP 2001-82304 A proposes the device for performing knock control in correspondence with changes in the ionic current produced by the fuel and the spark plug, but the ionic current is known to change in amount depending on the engine R.P.M., the load and the like, which the device in this publication does not consider. When considering an actual vehicle operation time, it is also necessary to learn the conditions mentioned above, and thus it is necessary to shorten the cycle for obtaining the average in the conventional example in the above-mentioned publication. When the cycle for obtaining the average is long, the operating conditions are such that the low R.P.M. is relatively frequent until the learning is completed. Then, immediately before the learning is completed, high R.P.M. is reached, and then when the learning ends, the learned value in which a proportion of the low R.P.M. is large is compared with the comparison level that should be used for comparison at the high R.P.M. time. Therefore, there is a fear that an appropriate correction amount cannot be set.
Further, even when the above-mentioned engine conditions are identical, the ionic current amount is known to vary at each ignition cycle. As such, in the case where the average is simply taken as in the conventional example of the above-mentioned publication, the fluctuations of the ionic current amounts from each ignition cycle are reflected in the learned value, and the results of the learning might not be stable.
Further, the ionic current amount at the time of misfire is extremely small, and it is known to become zero. However, in the conventional example of the above-mentioned publication, there is no consideration given to the misfire time. In a case where the misfire is frequent, the integral values of the ionic currents at the time of the misfire are reflected in the learned value, and it is possible that the results of the learning might not be stable.
The present invention has been made to solve the above-mentioned problems, and therefore has as an object to provide a knock control device for an internal combustion engine for accurately detecting fluctuation in an ionic current amount, to cope with change in the ionic current amount.
A knock control device of the present invention for an internal combustion engine comprises: ionic current detection means for detecting an ionic current generated immediately after ignition in a combustion chamber of an internal combustion engine; knock detection means for extracting a knock signal from the ionic current; and knock determination means for determining a knock status of the internal combustion engine based on the extracted knock signal, wherein the knock determination means includes comparison reference value setting means for comparing the knock signal outputted from the knock detection means with a filter value which has undergone filter processing; and wherein the knock control device for the internal combustion engine further comprises: control parameter correction request amount setting means for setting a control parameter correction request amount for correcting a control parameter at least including a retardation correction amount for retarding ignition timing of each cylinder, based on the comparison reference value set by the comparison reference value setting means, and the knock signal outputted from the knock detection means; control parameter correction means for correcting a control parameter for controlling ignition timing of an ignition device, based on the control parameter correction request amount that has been set; ionic current intensity determination means for determining ionic current intensity of the ionic current based on an output value from the ionic current detection means; and correction means for correcting at least one of the comparison reference value and the control parameter correction request amount, based on the ionic current intensity determined by the ionic current intensity determination means.